This invention relates to rotary airlock valves, and more particularly to vaneless rotary airlock valves.
Rotary airlocks have wide application in industry wherever dry free-flowing powders, granules, crystals or pellets are used. Airlocks are used on pneumatic conveying systems, dust control equipment, and as volumetric feeders for metering materials at precise flow rates from bins, hoppers, or silos into conveying or processing systems. The basic use of the airlock is as an airtight transition point, sealing pressurized systems against loss of air or gas while maintaining a flow of materials between components with different pressure.
Most conventional rotary airlocks conform to the basic design principle of a vaned rotor which revolves within a cylindrical casing. The problems associated with the vaned design are well known, principally being leakage, wear and product degradation.
The bladed wheel configuration of vaned airlocks is not geometrically conductive to sealing between the blades of the rotor and the insides of the casing. There are, therefore, two types of leakage inherent with vaned airlocks: static and dynamic. Static leakage is that which occurs between the rotor tips and the airlock casing, and accounts for 70 to 80 percent of total leakage. Static leakage (also called gap or clearance leakage) occurs whether or not the rotor is turning, and increases with wear of the airlock. Dynamic leakage is the amount of air or gas that fills the pockets of the rotor (the areas between the vanes of the rotor) as it revolves from the high pressure side to the low pressure side of the airlock. Dynamic leakage results from the pumping action of the airlock, and occurs only when the rotor of the airlock is turning. It accounts for 20 to 30 percent depending on pocket size and the speed at which the airlock turns. Neither static nor dynamic leakage can be disinherited from the generic vaned rotary airlock.
Major airlock damage occurs as a result of the gaps or clearances between the tips of the vanes of the rotor and the inside of the casing. The gaps permit static leakage through the airlock, which means that a continuous flow stream with varying levels of velocity is flowing between the tips of the vanes and the inside of the airlock casing. Jointly, the two surfaces that comprise the gap act as a rectangular orifice in restricting flow. The stream that is flowing through the created orifice is laden, in most operating conditions, with highly abrasive particles. The orifice created by the vane tips and the casing is, therefore, subjected to a continuous sandblasting effect. In varying lengths of time, the vanes and the casing become severely eroded from the sandblasting, rendering the airlock unusable and resulting in production loss and costly replacement of the airlock.
Product degradation takes different forms depending on the product and the process. The form of product degradation associated with conventional vaned rotary airlocks is the production of "fines." Fines are the small particles produced in vaned rotary airlocks as a result of material that becomes drawn into the gaps or clearances between the tips of the vanes of the rotor and the inside of the airlock casing. The rotating action of the vaned rotor crushes the material lodged in the gaps into smaller particles. Depending on the material being conveyed through the airlock, the production of fines may be excessive or relatively small. While fines are the desired end product of certain processes, they are undesirable and classified as rejected material in other processes. In any event, vaned rotary airlocks would not be the chosen item of equipment for the desirable production of fines.
As can be seen from the foregoing, conventional vaned rotary airlocks, due to their inherent leakage, wear and product degradation problems, do not properly fulfill their intended application function.